This invention relates to a method for making salts of hydroxy-substituted aromatic compounds. In one particular embodiment the invention relates to the preparation of alkali metal salts of hydroxy-substituted aromatic compounds.
Salts of hydroxy-substituted aromatic compounds find varied uses in the industry. They are frequently required for synthesis of hydroxy-substituted aromatic derivatives or for use as bases in chemical reactions. Various methods are known in the art for the preparation of salts of hydroxy-substituted aromatic compounds, more important among those being the methods of forming the dry salt.
U.S. Pat. No. 4,520,204 describes the manufacture of salts such as bisphenol A disodium salt by forming the salt in an aqueous solution with sodium hydroxide and adding the aqueous solution to boiling ortho-dichlorobenzene (oDCB) to dry the salt by azeotropically distilling water. The salt slurry in organic solvent is further dried by refluxing over calcium hydride. The distillate is refluxed over CaH2 and returned to the reaction vessel. The procedure is problematic in that foaming may occur during the addition of the aqueous salt solution to the refluxing ortho-dichlorobenzene. Further, this process requires careful attention in order to maintain the inventory of boiling oDCB during introduction of the aqueous solution.
U.S. Pat. No. 4,546,207 describes the manufacture of anhydrous salts of dihydroxy aromatic compounds by forming the salt in an aqueous solution with excess sodium hydroxide and then isolating the solid salt from the reaction mixture. The solid salt is then treated with an organic solvent and the mixture evaporated to dry the salt. With this procedure, the salt may be contaminated with excess sodium hydroxide, which interferes with subsequent reactions.
U.S. Pat. No. 4,257,953 describes a process for bisphenoxide salt preparation in which a bisphenoxide salt, e.g. aqueous bisphenol A disodium salt, is mixed with a solvent, e.g. toluene, and the water is removed by azeotropic distillation. However this process results in substantial accumulation of solid salt on the vessel walls.
U.S. Pat. No. 4,492,806 describes a process for bisphenol metal salt preparation and drying in which a solvent mixture comprising an aliphatic alcohol and a solvent such as toluene are mixed with an aqueous bisphenol metal salt solution, and the volatile species (alcohol and water) are azeotropically removed, yielding a dry fine slurry of bisphenol metal salt. Though this method is effective for bisphenol A disodium salt preparation, it requires an alcohol co-solvent, which introduces process complexity and also a flammable solvent.
Hence, there exists a need to provide alternate methods for preparing dry metal salts of hydroxy-substituted aromatic compounds.